More than 400 senior doctors and public health experts are calling on the House of Lords to throw out the government's health and social care bill, saying it will do "irreparable harm to the NHS, to individual patients and to society as a whole".

The signatories include Professor Sir Michael Marmot, the author of several reports on the links between wealth and health that suggest children born into poverty are penalised for life.

Marmot has until now not been openly critical of the coalition's approach, and instead has offered encouragement for David Cameron and Andrew Lansley's apparent enthusiasm for public health.

But Marmot and others in senior positions have now concluded the bill will damage all aspects of the health service.

"While we welcome the emphasis placed on establishing a closer working relationship between public health and local government, the proposed reforms as a whole will disrupt, fragment and weaken the country's public health capabilities," says the letter.

"The government claims that the reforms have the backing of the health professions. They do not. Neither do they have the general support of the public."

The letter details the harms the experts believe the health reform bill will do.

"It ushers in a significantly heightened degree of commercialisation and marketisation that will lead to the harmful fragmentation of patient care; aggravate risks to individual patient safety; erode medical ethics and trust within the healthcare system; widen health inequalities; waste much money on attempts to regulate and manage competition; and undermine the ability of the health system to respond effectively and efficiently to communicate disease outbreaks and other public health emergencies," the letter says.

In their judgment, the signatories say, the bill "will erode the NHS's ethical and co-operative foundations" and "will not deliver efficiency, quality, fairness or choice".

The signatories include around 40 directors of public health from around the country who have taken the difficult decision to go public with their concerns. There are also two senior members of the Faculty of Public Health, one of whom, Dr John Middleton, is a vice-president. Other well-known names include Professor John Ashton, director of public health in Cumbria, and Professor Michel Coleman from the London School of Hygiene and Tropical Medicine.

Dr David McCoy, consultant in public health medicine at the Inner North West London primary care trust, one of the organisers of the letter, said he was surprised at the number of people prepared to sign. "I think if we had continued to collect signatures, I'm quite sure we would have collected another 200 It is having a snowball effect," he said. "I think the feeling is incredibly strong."

There was a lot of debate about whether we should call for outright rejection or amendments, but there is a feeling the whole package of reforms is harmful and we need to express our position in the strongest terms. I think there was a feeling the forthcoming reading in the House of Lords is the last chance of minimising the harm and damage."

The public health community has not spoken out in this way before. "I think there has been an attempt to work with the reforms and work behind the scenes to optimise the proposed reforms," said Dr McCoy.

Dr Middleton said there was no great opposition to the planned move to place public health services such as smoking cessation within local authorities. "But the letter is a recognition from the public health community that the reforms proposed around the NHS are deeply damaging to the public health in themselves," he said. There was concern that they would lead to inequalities in healthcare and less access for the poorest and most deprived to the services they need.

"The experience of other countries that have 'liberated' their health systems has resulted in very poor health services for their communities. I'm thinking of Russia and China where a free market in health resulted in major falls in life expectancy and systems that had provided some safety net cover have failed," he said.

Commenting on the letter, published in the Daily Telegraph on the eve of health secretary Andrew Lansley's address to the Tory party conference, shadow health secretary John Healey said: "David Cameron is in denial, both about the damage his plans are doing to the NHS and the strength of opposition to his health bill.

"There is no mandate for the bill, either from the election or the coalition agreement. With the government having railroaded its plans through the Commons, heavy responsibility is now going to be shouldered by the Lords."

The Department of Health said: "Our plans to modernise the NHS will give patients more choice, root out waste and give trusted NHS staff more power to improve care.

"A group of the country's top doctors and NHS experts have already analysed the health bill and we have accepted all their recommendations to improve it. Many GPs and other doctors have also already spoken out in support."